A so-called smart key system that locks and unlocks vehicle doors through a wireless communication between an electronic key carried by a user and a vehicle side device mounted on a vehicle has been known. In the smart key system, when the vehicle doors are unlocked, the wireless communication is performed between the electronic key and the vehicle side device, and the vehicle side device needs to authenticate the electronic key. Since a distance range where the wireless communication can be performed is restricted to a relatively short distance, the electronic key is generally located in the vicinity of the vehicle side device.
However, when a malicious third party places a relay for relaying signals of the wireless communication between the electronic key and the vehicle side device, even if the electronic key is present in a position remote from the vehicle, the electronic key and the vehicle side device can wirelessly communicate with each other. As a result, the vehicle doors are likely to be unlocked contrary to the intention of the user. A technique in which the wireless communication between the electronic key and the vehicle side device is performed through such a relay is generally called “relay attack”.
As a device for preventing the vehicle doors from being unlocked due to the relay attack, a device disclosed in, for example, Patent Literature 1 has been known. In the device disclosed in Patent Literature 1, each of the electronic key and the vehicle side device includes a GPS receiver. The electronic key transmits a signal including location information on the electronic key to the vehicle side device. The vehicle side device permits the vehicle doors to be unlocked on the condition that the location information of the electronic key matches the vehicle location information.
However, in the device of Patent Literature 1, each of the electronic key and the vehicle side device needs to have the GPS receiver for acquiring the location information. For that reason, the configuration of the overall device becomes complicated, resulting in a fault that the costs increase.